1. Field of the Invention
The present invention relates to a method and device for adjusting a white balance in a color image of a subject which is picked up by an image pickup device having a blue (B) sensor, a green (G) sensor, and a red (R) sensor, such as a digital camera. The present invention further relates to an image pickup device having a blue (B) sensor, a green (G) sensor, and a red (R) sensor, such as a digital camera, and being suitable for utilizing the above method and device.
2. Description of the Related Art
Many color negative films are composed of sensitive materials having such spectral sensitivities that an appropriate white balance is achieved under daylight of 5,500 K color temperature. When images are picked up under fluorescent light by using the above color negative films, greenish images are obtained. This is because the red photosensitive layers of such color negative films have relatively high spectral sensitivities on the range of longer wavelengths for separating red and green in reproduced images, and because the spectral energy of the fluorescent light is relatively low in a certain wavelength range between the blue and green wavelengths as illustrated in FIGS. 5 to 7. In order to solve the above problem, a special color negative film is proposed. In the color negative film, the high-sensitivity spectral portion of the red photosensitive layer is shifted toward the shorter wavelength side, and the fourth photosensitive layer having high spectral sensitivity in a certain wavelength range between the blue and green wavelengths is provided. By using such a color negative film, images having an appropriate white balance can be obtained under fluorescent light as well as daylight.
On the other hand, in the digital electronic still cameras (hereinafter called digital cameras), images picked up by a pickup device such as a charge-coupled device (CCD) is stored as digital image data in a storage medium such as an IC card or an internal memory provided in the digital camera. The picked up image can be reproduced based on the stored digital image data as a hard copy such as a photoprint, or a soft copy on a display screen. In this case, the white balance in the entire image is adjusted so that an image of a gray portion of a subject is reproduced in gray in the photoprint or on the display screen.
However, even in the digital cameras, the output of the image pickup device is usually adjusted so that an appropriate white balance is achieved under daylight. Therefore, when an image is picked up under fluorescent light, the white balance of the image is different. The levels of red (R) signal are sufficient since charge-coupled devices (CCD) in the digital cameras can be set so that high-sensitivity spectral portions of R sensors constituting the CCD is located on the shorter wavelength side than the high-sensitivity spectral portions of the red photosensitive layers in the color negative films. Nevertheless, under fluorescent light, the levels of blue (B) signals are relatively low compared with green (G) signals and red (R) signals. Therefore, when an image is picked up under fluorescent light by using a digital camera the output of which is adjusted under daylight, the image becomes yellowish. In this case, an image having an appropriate white balance can be obtained by adjusting the white balance based on the digital image data obtained by the digital camera.
However, when an image is picked up under mixed illumination of daylight and fluorescent light, it is difficult to achieve a white balance in the image, which is appropriate for both of the color temperatures of daylight and fluorescent light, since an appropriate white balance can be achieved for only one of the fluorescent light and the daylight.
The first object of the present invention is to provide a method for achieving an appropriate white balance in an image picked up under fluorescent light or mixed illumination of daylight and fluorescent light.
The second object of the present invention is to provide a device for achieving an appropriate white balance in an image picked up under fluorescent light or mixed illumination of daylight and fluorescent light.
The third object of the present invention is to provide an image pickup device in which an appropriate white balance can be achieved in an image picked up under fluorescent light or mixed illumination of daylight and fluorescent light.
(1) According to the first aspect of the present invention, there is provided a method for adjusting a white balance of an image represented by a first (blue), second (green), and third (red) signals which are respectively produced by a first (blue), second (green), third (red) sensors. The method comprises the steps of (a) obtaining the first (blue), second (green), and third (red) signals and a fourth signal, where the fourth signal is produced by a fourth sensor, the first (blue), second (green), and third (red) sensors are sensitive to light in a blue, green, and red wavelength ranges, the fourth sensor is sensitive to light in a certain wavelength range located between a first and second wavelengths, a spectral sensitivity distribution of the first (blue) sensor has a peak at the first wavelength, and a spectral sensitivity distribution of the second (green) sensor has a peak at the second wavelength; (b) modulating the first (blue) signal according to an absolute value of the difference between the first (blue) and fourth signals, and outputting a modulated signal; and (c) obtaining a white balance based on color image data comprised of the modulated signal and the second (green) and third (red) signals.
As mentioned before, the spectral energy of the fluorescent light is relatively low in a wavelength range between the blue and green wavelengths. Since the fourth sensor is sensitive to light in a certain wavelength range located between the peaks of the spectral sensitivities of the first (blue) and second (green) sensors, the level of the first (blue) signal is greater than the level of the fourth signal. Therefore, when the level of the first (blue) signal is modulated with the absolute value of the difference between the first (blue) and fourth signals, the level of the modulated signal can be made greater than the original first (blue) signal.
Spectral energy of some fluorescent light may not be relatively low in a certain wavelength range between the blue and green wavelengths. Even in this case, as long as a substantial difference exists between the first (blue) and fourth signals, the level of the modulated signal can be made greater than the original first (blue) signal. Therefore, when the white balance of the image represented by the second (green) and third (red) signals and the modulated signal is adjusted, the use of the modulated signal, instead of the first (blue) signal, compensates for the insufficiency of the original first (blue) signal, and consequently an image having an appropriate white balance can be obtained corresponding to the image which is originally represented by the first (blue), second (green), and third (red) signals. That is, the resultant image in which the white balance is adjusted as above does not become yellowish even when the image is picked up under fluorescent light.
(2) According to the second aspect of the present invention, there is provided a method for adjusting a white balance of an image represented by a first (blue), second (green), and third (red) signals which are respectively produced by a first (blue), second (green), third (red) sensors. The method comprises the steps of (a) obtaining the first (blue), second (green), and third (red) signals and a fourth signal, where the fourth signal is produced by a fourth sensor, the first (blue), second (green), and third (red) sensors are respectively sensitive to light in a blue, green, and red wavelength ranges, the fourth sensor is sensitive to light in a certain wavelength range located between a first and second wavelengths, a spectral sensitivity distribution of the first (blue) sensor has a peak at the first wavelength, and a spectral sensitivity distribution of the second (green) sensor has a peak at the second wavelength; (b) modulating the first (blue) signal according to an absolute value of the difference between the first (blue) and fourth signals, and outputting a first modulated signal; (c) modulating the third (red) signal according to an absolute value of the difference between the third (red) and fourth signals, and outputting a second modulated signal; and (d) obtaining a white balance based on color image data comprised of the first and second modulated signals and the second (green) signal.
When the white balance is adjusted based on the first (blue) signal, the sensitivity to light in the red wavelength range is sufficient even under fluorescent light. However, when the white balance is adjusted based on the (first) modulated signal and the second (green) and third (red) signals, sometimes the sensitivity to light in the red wavelength range becomes considerably smaller than the sensitivities to light in the blue and green wavelength ranges. In the method according to the second aspect of the present invention, the first (blue) signal is modulated with the absolute value of the difference between the third (red) and fourth signals to produce the second modulated signal, and the white balance is adjusted based on the first and second modulated signals and the second (green) signal. Since the level of the second modulated signal can be made greater than the original third (red) signal, the level of the second modulated signal can be balanced with the levels of the modulated B signal and the G signal. Therefore, the white balance can be adjusted based on the balanced color signals.
(3) According to the third aspect of the present invention, there is provided an image pickup device comprising a first (blue) sensor which is sensitive to light in a blue wavelength range, and outputs a first (blue) signal; a second (green) sensor which is sensitive to light in a green wavelength range, and outputs a second (green) signal; a third (red) sensor which is sensitive to light in a red wavelength range, and outputs a third (red) signal; and a fourth sensor which is sensitive to light in a certain wavelength range between a first and second wavelengths, and outputs a fourth signal, where a spectral sensitivity distribution of the first (blue) sensor has a peak at the first wavelength, and a spectral sensitivity distribution of the second (green) sensor has a peak at the second wavelength.
(i) The image pickup device according to the third aspect of the present invention may further comprise a modulation unit which modulates the first (blue) signal according to an absolute value of the difference between the first (blue) and fourth signals, and outputs a modulated signal, and a white balance adjustment unit which adjusts a white balance in an image represented by the first (blue), second (green), and third (red) signals, based on color image data comprised of the modulated signal and the second (green) and third (red) signals.
(ii) The image pickup device according to the third aspect of the present invention may further comprise a first modulation unit which modulates the first (blue) signal according to an absolute value of the difference between the first (blue) and fourth signals, and outputs a first modulated signal, a second modulation unit which modulates the third (red) signal according to an absolute value of the difference between the third (red) and fourth signals, and outputs a second modulated signal, and a white balance adjustment unit which adjusts a white balance in an image represented by the first (blue), second (green), and third (red) signals, based on color image data comprised of the first and second modulated signals and the second (green) signal.
(4) According to the fourth aspect of the present invention, there is provided a white balance adjusting device for adjusting a white balance of an image represented by a first (blue), second (green), and third (red) signals which are respectively produced by a first (blue), second (green), third (red) sensors. The white balance adjusting device comprises a signal input unit which inputs a first (blue), second (green), third (red), and fourth signals respectively produced by a first (blue), second (green), third (red), and fourth sensors, where the first (blue), second (green), and third (red) sensors are respectively sensitive to light in a blue, green, and red wavelength ranges, and the fourth sensor is sensitive to light in a certain wavelength range located between a first and second wavelengths, a spectral sensitivity distribution of the first (blue) sensor has a peak at the first wavelength, and a spectral sensitivity distribution of the second (green) sensor has a peak at the second wavelength; a modulation unit which modulates the first (blue) signal according to an absolute value of the difference between the first (blue) and fourth signals, and outputs a modulated signal; and a white balance adjustment unit which obtains a white balance based on color image data comprised of the modulated signal and the second (green) and third (red) signals.
(5) According to the fifth aspect of the present invention, there is provided a white balance adjusting device for adjusting a white balance of an image represented by a first (blue), second (green), and third (red) signals which are respectively produced by a first (blue), second (green), third (red) sensors. The white balance adjusting device comprises a signal input unit which inputs a first (blue), second (green), third (red), and fourth signals respectively produced by a first (blue), second (green), third (red), and fourth sensors, where the first (blue), second (green), and third (red) sensors are respectively sensitive to light in a blue, green, and red wavelength ranges, and the fourth sensor is sensitive to light in a certain wavelength range located between a first and second wavelengths, a spectral sensitivity distribution of the first (blue) sensor has a peak at the first wavelength, and a spectral sensitivity distribution of the second (green) sensor has a peak at the second wavelength; a first modulation unit which modulates the first (blue) signal according to an absolute value of the difference between the first (blue) and fourth signals, and outputs a first modulated signal; a second modulation unit which modulates the third (red) signal according to an absolute value of the difference between the third (red) and fourth signals, and outputs a second modulated signal, and a white balance adjustment unit which obtains a white balance based on color image data comprised of the first and second modulated signals and the second (green) signal.
In the first to fifth aspects of the present invention, it is preferable that the fourth sensor is sensitive to light in the wavelength range of 460 to 535 nm. It is further preferable that the fourth sensor is sensitive to light in the wavelength range of 475 to 535 nm.